It has been recognized that due to environmental concerns and newly enacted rules and regulations, saleable products must meet lower and lower limits on contaminants such as sulfur and nitrogen. Recently, new regulations require the essentially complete removal of sulfur from liquid hydrocarbons which are used in transportation fuels, such as gasoline and diesel.
Furthermore, there is increasing pressure to improve the quality of diesel fuel making it more difficult to blend light cycle oil (LCO) from a fluid catalytic cracking unit (FCC) into the diesel pool. Pressure to increase cetane number and reduce particulate emissions from diesel engines makes it more difficult to blend LCO, as the LCO stream contains a high proportion of aromatic compounds, naphthalenes, indanes, and tetralins, all of which have poor cetane numbers and tend to produce soot on burning. As the proportion of LCO that can be blended into diesel is reduced, the remainder must be blended into heavy fuel oil and as a result its value is substantially reduced.
An alternative means for using LCO is to subject it to mild hydrocracking to improve the product quality. Hydrocracking of LCO produces an aromatic-rich gasoline stream of higher octane than would normally be obtained from hydrocracking, together with a distillate stream of improved cetane number. In some refineries configured for petrochemical production, it may be desirable to carry out additional processing to maximize the yield of valuable xylenes from the aromatic gasoline produced in the LCO hydrocracker.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from a large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. The xylenes are not directly recovered from petroleum by the fractionation of naphtha in sufficient volume to meet demand nor in a high enough purity; thus conversion of other hydrocarbons is necessary to increase the purity and yield of the xylenes. For straight run naphtha feedstocks, which may be naphtha distilled out of crude oil, it is necessary to utilize high severity reforming with inter-reactor reheat to convert large amounts of paraffins, such as from about 40 to about 70 weight percent, and having about 30 to about 60% total cyclic content, to the desired xylenes and/or benzene. Moreover, the large amount of non-aromatic content remaining in the reformed naphtha requires substantial subsequent processing to remove the non-aromatics and to transalkylate the aromatics to benzene and xylene.
Heretofore where just the non-xylene co-boiling feedstock streams having C7 and C9/C10 cuts from naphtha might be fed to a transalkylation zone and xylene co-boiling streams containing C8 cuts in the naphtha may be bypassed from the transalkylation zone and fed directly to a product xylene column, the xylene product purity becomes low, i.e., less than 80% xylene content.